High power electron, ion and other particle beams are used in a variety of medical, scientific, and industrial applications including, for example, the use of electron beams to cure various polymers and other composites. One problem which has hindered the further application of such high power beams is the difficulty of extracting the beam from the vacuum environment in which they are normally generated to atmosphere where the beams are used for most applications.
Heretofore, two techniques have typically been used for accomplishing such beam extraction, each of which has severe limitations. The first technique uses a thin foil window, with titanium being a typical material used for such windows. While such windows are compact and do not pose limitations on the diameter of the beam being extracted, the high pressure differential across these windows, which is roughly 14 psi, requires a thickness of window and the use of material having sufficient strength, and in particular sufficient burst strength or ultimate tensile strength at high temperature, so as to be able to function at such a pressure differential without bursting. However, the thickness of such foils and the materials such as titanium which are required to achieve an ultimate tensile strength sufficient to support a 14 psi pressure differential results in significant heating of the window during high power extraction as the window absorbs energy from the beam. This heating can lead to catastrophic implosion. Attempts have been made to control the heating of the foil by providing water cooling around the periphery. However, the foils remain difficult to cool and this has meant that the amount of beam power per unit area which can be transmitted through the foil window is limited. This is a particular problem when dealing with low energy beams (i.e. less than 500 keV) because low energy beams deposit a higher fraction of their power on the foil windows than do higher energy beams. However, low energy beams are desirable for many application. Further, the problem of window heating significantly shortens the useable life of a window, resulting in the need for more frequent window replacement, either because of window failure, or preferably through routine maintenance. Since the system cannot be used during window replacement, and since window replacement is a relatively difficult and time consuming procedure because of the large pressure differentials across the window, the need for frequent window replacements signficantly increases the cost of operating the beam generating equipment and signficantly reduces the efficiency with which such equipment can be used.
The second technique is referred to as differential pressure extraction or more generally as air dynamic window extraction. These techniques involve passing the beam through successive differentially pumped chambers to maintain the desired vacuum in the beam generation chamber. The advantage of these techniques is that they place no limit on the amount of power which can be extracted. However, each pumping stage takes up a significant space, particularly in the direction of the beam, and the number of pumping stages required to get from vacuum to atmospheric pressure can be substantial, resulting in a relatively long and cumbersome device which can also be relatively expensive to operate. However, the most significant limitation on the systems is that, since the pumping rate is proportional to aperture diameter, the systems as a practical matter are limited to relatively small diameter beams (approximately 2 mm or less).
A need therefore exists for an improved extraction system for use with high power beams in general, and with high power, low energy electron beams in particular, which does not impose significant limitations on either the power of the beam being extracted or on the diameter of the extracted beam, while not being unduly cumbersome or expensive to operate. Such a system should also not impose an undue maintenance burden.